In this disclosure colour means all colours including black and white and all shades of grey. Nowadays, a large variety of digital multi-colour reproduction systems exist. Usually a distinction is made between these systems based on the kind of marking particles used, e.g. ink or toner, the imaging process employed, e.g. magnetography, or electro(photo)graphy, or inkjet, the productivity or the media range.
A distinction can also be made dependent on how the multi-colour image of marking particles is composed. For instance, the multi-colour image of marking particles may be composed of a plurality of registered colour separation images where the marking particles of the respective colours associated with the respective colour separation images are superimposed. This approach has some inherent disadvantages. Firstly, because the marking particles of the different colours are superimposed, the total marking particles pile height can be high, particularly in full colour high density image areas. Besides the fact that a high total marking particles pile height is noticeable to the customer both visually and palpably, this may also negatively influence medium curl and transport as well as reduce the resistance against external mechanical influences such as scratches and folding. Moreover, different image compositions, e.g. different density and/or colour composition, lead to topographic differences on the medium which reinforce some of the above mentioned disadvantages and reveal additional disadvantages, such as gloss differences between different image parts. These disadvantages are particularly observable when toner particles are employed as marking particles because the size of toner particles is typically in the micrometer range.
Systems generating superimposed multi-colour images usually employ a limited number of process colours, i.e. typically four colours are used which are black, magenta, cyan and yellow, where an increased number of process colours also may increase the maximum marking particles pile height.
Alternatively, the multi-colour image of marking particles may be composed of a plurality of registered colour separation images where the marking particles of the respective colours associated with the respective colour separation images are positioned contiguous to each other. This approach has the advantage of a limited marking particles pile height and imposes no limitation on the number of process colours.
Therefore, in principle, digital systems capable of producing such images have the inherent advantage of digitally reproducing images with a “look and feel” comparable to images reproduced by offset lithography, which is the quality reference for the customer. In such a system, the digital images are first decomposed into a selection of process colours of the system yielding a number of digital colour separation images. The process colours can be any colour available in the system such as e.g. black, white, cyan, magenta, yellow, red, green and blue. The respective digital colour separation images are sequentially converted in register into colour separation images of marking particles of the respective associated colour on a moving image-carrying member so as to form registered composite multi-colour images of coloured marking particles on the image-carrying member.
A possible disadvantage of the latter system is its sensitivity with respect to register errors. A register error occurs when at least two colour separation images are formed on the moving image-carrying member with a displacement relative to one another. There are many possible causes for such a displacement including, but not limited to, speed variations of the moving image-carrying member, mechanical tolerances in the parts of the system, wear and/or synchronization errors in the moving parts of the system. There are different possible appearances of such register errors in multi-colour images on the image-carrying member and by consequence also on the medium. For instance, as a result of a register error, adjoining image elements being part of different colour separation images may overlap each other on the image-carrying member yielding superimposed marking particles of the respective colours instead of contiguous ones.
To limit the visual appearance of such an error, preferably the formation process of the respective colour separation images on the image-carrying member is complementary. This means that the image formation process is such that in the process sequence marking particles of a particular colour are only accumulated on the free surface of the image-carrying member and not on coloured marking particles which are already accumulated on the image-carrying member in preceding steps. Although the visual appearance of the register error is suppressed, due to a register error for instance image pixels of different separation images partially or completely corresponding to the overlapping region of adjacent image elements on the image-carrying member are rendered inadequately leading to colour differences between the digital image and the image of coloured particles on the image-carrying member. In practice, it is observed that the register errors are much greater in the propagation direction of the image-carrying member than in the direction perpendicular thereto.